There is a need for pre-loaded compression springs in many mechanical devices or equipment in many different fields. Wherever a mechanism or component must be held in an accurate position against an opposing force (e.g., gravity, air pressure, a spring, etc.) there is a potential need for a pre-loaded compression spring. In some situations it may be posible to use fixed stops to meet requirements, but in many situations this is either impossible or impractical.
A conventional spring is often used to press an assembly against a stop in order to locate the assembly accurately. However, in order to produce adequate spring force under compression, it is often necessary for the spring to be quite long in its free standing state. Thus, in order to effectively use such a spring for this purpose, the design of the assembly must be such that the spring does not produce mechanical interference in the setting in which it is used.
In some situations it is necessary for the spring to be pre-loaded in compression to enable it to fit into the space available. Previously proposed pre-loaded springs have not been satisfactory for all purposes.
Conventional pre-loaded spring designs typically include a helical compression spring constrained to a pre-loaded height by means of a solid rod or post. The means for pre-loading the springs usually consists of parts which extend through the ends of the assembly and have nuts, circlips, or other mechanical means for securing them on the outside. It is difficult and time-consuming to assembly and disassemble such devices, especially since the spring pre-load must be overcome before the pre-loading means can be assembled or disassembled.
Because of the design, the length of height of the prior assembly remains the same when the spring is compressed further. As a result, there is a certain amount of space which is required in order to permit use of such devices. Although it is sometimes possible to place one end of such a device into a counter-bored hole to reduce the effective height of the device, this is not always possible or practical. For example, in some situations there is insufficient space available to permit counter-boring. Also, this procedure adds cost and may be complicated to effect.
A pre-loaded spring device of the type described above, as used in circuit board testing apparatus, is described in U.S. Pat. No. 4,701,700. Because such device has a finite height, thered are inherent limitations associated with the use of such device. For example, the degree of compression of the spring is limited by the effective length of the screw fastener. Also, such device penetrates one of the plate members which supports the device.
There has not heretofore been provided a pre-loaded compression spring assembly which produces significant force at a pre-loaded height, has fairly long travel, has a short overall height when fully compressed, requires minimal area, and is easy to mount into a mechanical assembly.